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Article #11:  Flow Gauge Conversions

October 16, 2006
In each of our Tip of the Week articles we have tried to provide valuable information about the function and use of our Flow Finders™ and Flow Gauge™. The more informed you are in regard to these measurements tools, the more efficient you will become when measuring flow. One situation in which the accuracy of Flow Gauge readings can be affected is when an air pipe system experiences major drops in pressure. Therefore, in this Tip of the Week we'll discuss how low pipe pressure can impact Flow Finder readings.

As you're probably aware, an air flow reading can be determined by using System Studies' Flow Gauge in conjunction with a Flow Finder. The Flow Gauge converts the pressure differential created by the Flow Finder's calibrated orifice into an accurate flow rate. The flow rate indicated by the Flow Gauge is based on a pipe pressure of 9.0 Pounds per Square Inch (PSI). Therefore, a reading taken by a Flow Gauge in an air pipe with 9.0 PSI will always be accurate. If the pipe pressure is over or under 9.0 PSI, however, readings will be influenced.

For example, if a Flow Gauge indicates a flow reading of 50 Standard Cubic Feet per Hour (SCFH) and the pipe pressure is 6.0 PSI, the Flow Gauge reading will be higher than the actual or true air flow rate. Why? When the pipe pressure is 6.0 PSI, air molecules are less compressed, enabling them to pass through the Flow Finder orifice at an accelerated rate. And remember, the Flow Gauge is calibrated to read pressure differential at 9 PSI. So, a lower pipe pressure at the Flow Finder location would mean that the flow rate indicated by the Flow Gauge is somewhat higher than the actual reading. Conversely, a pipe pressure of 11 PSI, for example, would result in more densely compressed air molecules forcing their way through the orifice. This results in a lower Flow Gauge reading when compared with the true flow rate.

To help determine accurate flow readings with the Flow Gauge at different air pipe pressures, we have compiled the conversion chart on the next page. You can see that a Flow Finder reading taken at an air pipe that has a delivery pressure of 6.0 PSI requires an offset calculation of 0.87. So, if the Flow Finder reading is 50 SCFH, the actual or accurate flow reading will be 43.5 SCFH (50 x .87 = 43.5 SCFH). It is important to note that, in most cases, the conversion process will be unnecessary except in pipes that have significant changes in pressure and/or high flow rates.

See Conversion Chart Below.

Air Pipe Pressure Multiplier
12 1.13
11 1.08
10 1.04
9 1.00
8 0.96
7 0.92
6 0.87
5 0.83
4 0.79
3 0.75
2 0.70
1 0.66

Hopefully, this article has explained how it could be possible for one Flow Finder at a pipe alarm panel to read 60 SCFH (with an input pressure of 9 PSI) and another Flow Finder installed in the air pipe at a manifold location to read 65 SCFH (with an input pressure of 7 PSI). The difference in readings is the result of air pressure, not flow. Using the table above, you can see that the converted flow rate for the Flow Finder at the manifold location would be 59.8 SCFH. This reading accurately represents the true flow rate at this location.

If you have any questions regarding Flow Gauge conversion, or if you would like any additional information on the Flow Finders and Flow Gauge, please give us a call or drop us some email.